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copied!<p>Because the writer of Baseclass has explicitly declared that Bar has to be a read-only property. It doesn't make sense for derivations to break this contract and make it read-write.</p> <p>I'm with Microsoft on this one.<br> Let's say I'm a new programmer who has been told to code against the Baseclass derivation. i write something that assumes that Bar cannot be written to (since the Baseclass explicitly states that it is a get only property). Now with your derivation, my code may break. e.g.</p> <pre><code>public class BarProvider { BaseClass _source; Bar _currentBar; public void setSource(BaseClass b) { _source = b; _currentBar = b.Bar; } public Bar getBar() { return _currentBar; } } </code></pre> <p>Since Bar cannot be set as per the BaseClass interface, BarProvider assumes that caching is a safe thing to do - Since Bar cannot be modified. But if set was possible in a derivation, this class could be serving stale values if someone modified the _source object's Bar property externally. The point being '<em>Be Open, avoid doing sneaky things and surprising people</em>'</p> <p><strong>Update</strong>: <em>Ilya Ryzhenkov asks 'Why don't interfaces play by the same rules then?'</em> Hmm.. this gets muddier as I think about it.<br> An interface is a contract that says 'expect an implementation to have a read property named Bar.' <strong>Personally</strong> I'm much less likely to make that assumption of read-only if I saw an Interface. When i see a get-only property on an interface, I read it as 'Any implementation would expose this attribute Bar'... on a base-class it clicks as 'Bar is a read-only property'. Of course technically you're not breaking the contract.. you're doing more. So you're right in a sense.. I'd close by saying 'make it as hard as possible for misunderstandings to crop up'.</p>

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